Diff tells what lines must be changed in two files to bring them
into agreement. If one file is a directory, then a file in that
directory with basename the same as that of the other file is
used. If both files are directories, similarly named files in
the two directories are compared by the method of diff for text
files and
cmp(1) otherwise. If more than two file names are given, then
each argument is compared to the last argument as above. The –r
option causes diff to process similarly named subdirectories recursively.
When processing more than one file, diff prefixes file differences
with a single line listing the two differing files, in
the form of a diff command line. The –m flag causes this behavior
even when processing single files.

The normal output contains lines of these forms:

n1an3,n4
n1,n2dn3
n1,n2cn3,n4

These lines resemble ed commands to convert file1 into file2.
The numbers after the letters pertain to file2. In fact, by exchanging
`a' for `d' and reading backward one may ascertain equally how
to convert file2 into file1. As in ed, identical pairs where n1
= n2 or n3 = n4 are abbreviated as a single number.

Following each of these lines come all the lines that are affected
in the first file flagged by `<', then all the lines that are affected
in the second file flagged by `>'.

The –b option causes trailing blanks (spaces and tabs) to be ignored
and other strings of blanks to compare equal. The –w option causes
all white–space to be removed from input lines before applying
the difference algorithm.

The –n option prefixes each range with file: and inserts a space
around the a, c, and d verbs. The –e option produces a script of
a, c and d commands for the editor ed, which will recreate file2
from file1. The –f option produces a similar script, not useful
with ed, in the opposite order. It may, however, be useful as
input to a stream–oriented post–processor.

The –c option includes three lines of context around each change,
merging changes whose contexts overlap. In this mode, diff prints
– and + instead of < and > because the former are easier to distinguish
when mixed. The –a flag displays the entire file as context.

Except in rare circumstances, diff finds a smallest sufficient
set of file differences.